Ultra-short-wave dielectric heating equipment



March 14, 1967 KlYosHl UCHIMARU ET AL 3,309,489

ULTRASHORT-"WAVE DIELECTRIC HEATING EQUIPMENT Filed July 20, 1965 United States Patent C 3,309,489 ULTRA-SHORT-WAVE DIELECTRIC HEATING EQUIPMENT Kiyoshi Uchimaru and Yoshiaki Shimano, both of Tokyo, Japan, assignors to Nippon Electric Company Limited,

Tokyo, Japan, a corporation of Japan Filed July 20, 1965, Ser. No. 473,326 Claims priority, application Japan, Mar. 27, 1962,

37/ 12,268 3 Claims. (Cl. 219--10.75)

The instant invention relates to ultra-short-wave oscillator means and more particularly to ultra-short-wave oscillator equipment for use in heating dielectric materials at high efficiency through the employment of ultra-short-waves, and is a continuation-in-part of copending application Ser. No. 267,716 led Mar. 25, 1963, entitled, Ultra-Short-Wave. Dielectrics Heating Equipment, now abandoned.

Present day dielectric heating apparatus frequently employ resonant circuits comprised of a coil and a condenser which have been used in systems up to the frequency of several tens of megacycles as ultra-short-wave oscillator means which employ industrial transmitting tubes. The power absorbed (Pab) by a dielectric material which is inserted between a pair of electrodes is proportional to the frequency (f) of the applied waves, the square of the electric eld intensity (E) to which the dielectric is subjected, and the loss factor (K) of the dielectric material, i.e., Pab ofKfEz. The rate at which heat is generated in a dielectric is proportional to its loss factor which is equal to the product of its dielectric constant and its power factor. Both the dielectric constant and power factor are Vfunctions of frequency, therefore, the loss factor changes (increases) with changing (increasing) frequency.

Therefore, in order to perform suicient heating, both the frequency and the electric field intensity must be increased to compensate for a decrease in the loss factor of the dielectric material. The electric field intensity, however, cannot be increased indefinitely since there is an upper limit imposed by the presence of the puncture voltage. If this upper limit is exceeded the breakdown of the dielectric or an insulation failure due to spark discharge through the air between the dielectric surface and either electrode surface occurs. This phenomenon can be likened to a puncturing action, hence the adoption of the terminology puncture voltage.

Once this puncture voltage limit is reached, the only remaining variable which may be altered is the frequency. Thus, the frequency of the applied electric Wave must be increased if, after reaching the puncture voltage limit, the power absorbed by the dielectric is still insuflcient for the heating requirements.

Increasing the frequency of the applied wave contracts the heating time interval and enables a dielectric of a low loss factor or a thin dielectric to be heated by the ultra-short-wave heating equipment. Operation at increasing frequencies, however,.necessitates the reduction in size of both the coil and the condenser employed in the oscillatory circuit. This size reduction, in turn, increases the circuit losses and decreases, therefore, the efficiency of the heating operation. For this reason, it has become necessary to abandon the coil and capacitor oscillatory circuit and substitute in its stead a transmission line or a coaxial line which is employed as a resonant line for operation at the desired oscillating frequencies.

It has been conventional in ultra-short-wave heating facilities to employ an ultra-short-wave electron tube oscillator having a one-quarter wavelength shorted line wherein the current is short-circuited by a con- 3,309,489 Patented Mar. 14, 1967 ice denser structure which is connected at the opposite end of the resonant line. With such an arrangement the dielectric to be heated is inserted between a pair o-f electrocles in the separately installed secondary circuit so as to be capacitively or inductively coupled to the resonant line. When a dielectric material is connected in series in such a circuit, initiation of oscillation becomes diicult because the impedance as viewed from the anode and grid of the electron tube is too high. Further, with such an arrangement the Q of the secondary circuit tends to become excessive because the dielectric power factor of the dielectric material being heated is normally small. For this reason the operation of the oscillator becomes unstable due to the phenomenon accompanied by frequency skip or changes in output, that is, the pulling phenomenon. This phenomenon is an indication of the oscillator stability and is identified by the pulling figure which is equal to the maximum change in frequency encountered when the oscillator load is changed so as to vary the voltage standing wave ratio of 1.5 through all phase angles. Thus, the addition of the capacitive coupling of the secondary circuit substantially increases the resonant line capacitance which necessitates the decrease in the length of the resonant line in order to adjust the circuit to operate at a desired frequency. Also, the electron tube lead inductances, interelectrode capacitances and the lead inductances of the terminals connecting the electron tube to the resonant line further act to alter the inductance and capacitance parameters of the resonant line and to further require contraction of the length of the resonant line. The minimum length of the resonant line is that length at which the anode and grid electrodes of the tube at the envelope Wall thereof are short-circuited and this length determines the upper limit of the oscillator frequency for dielectric heating purposes.

One alternative method employed in conventional apparatus is to cause oscillation by use of a quarter-wavelength line coupled between the anode and grid of the electron tube, with the far end of the line being shortcircuited. A secondary circuit is electromagnetically coupled to the quarter-wavelength line with the secondary circuit being provided with means `for inserting a dielectric load to be heated. Still another method is that of connecting a quarter-wavelength line between the electron tube anode and grid, with the far end of the line being short-circuited and to install a pair of electrodes at a comparatively high distribution voltage point along said quarter-wavelength line and inserting a dielectric material to be heated between the said pair of electrodes.

In order to raise the generated frequency by the latter alternative method, the line length must be diminished. When the physical dimensions of the electrodes which receive the dielectric load are taken into consideration, the high voltage distribution point moves so close to the electron tube envelope that there is no satisfactory way of physically connecting the electrodes to the electron tube, thus limiting the upper oscillation frequency to a point that the maximum frequency obtainable through the use of this circuit will not provide suicient heating.

Considering the former of the two alternative methods, the physical dimensions of the electrodes which receive the dielectric load to be heated need not be taken into consideration with respect to ,the generated frequency, as compared with the latter of the two alternative methods. In this case, the shortest length of the primary circuit short-circuiting the anode in the grid determines the upper frequency limit. Since the value of Q of the load (the dielectric material) itself is quite high, the loaded Q of the secondary circuit becomes generally higher than the loaded Q of the primary circuit, resulting in the so-called pulling phenomenon of frequency skipping or unstable operating conditions due to changes in the load which occur in the course of heating, as was previously described. This makes the former of the two alternative approaches extremely disadvantageous.

The dielectric heating str-ucture of the instant invention provides the use of a resonant line which can be made appreciably long, while at the same time enabling the generation of the applied wave to occur at the maximum frequency limited only by the physical dimensions of the electron tube and without imposing or subjecting the restrictions of the external circuitry connected to the electron tube. Therefore, the upper limit of oscillating frequency is limited only by the practical limits of the electron tube and not by the limits of the external circuitry. In addition thereto, the oscillator means can be loaded without worry of either inductively or capacitively coupling the secondary circuit to the oscillator means, thereby providing an ultra-short-wave oscillator providing an extremely stabilized heating operation.

The instant invention is comprised of a resonant line having one end connected to the transmitting tube electrodes. The opposite end is substantially open-circuited and is further provided with suitable electrodes for the receipt of the dielectric material therebetween. An air gap of suitable dimensions is provided between the dielectric medium and the electrodes in order to insure a substantially open-circuited condition at the electrode terminals.

A resonant line is provided having a first end thereof connected to the pair of electrodes adapted for receiving the dielectric material to be heated. The second end thereof is electrically connected to the electrodes of the transmitting tube. The electrodes receiving the dielectric material are positioned relative to the dielectric material and relative to one another so as to be substantially open-circuited. Such an arrangement permits insertion or loading of the dielectric receiving electrodes without affecting the operating frequency of the `half-wave line. In addition thereto, the end of the resonant line connected to the transmitting tube electrodes is not significantly affected by the lead inductances and inter-electrode capacitances of the electron tube or the inductances of leads connecting the electrodes of the electron tube to the resonant line so as to permit the use of a substantially lengthened resonant line over prior-art devices.

The position at which the electrodes which receive the dielectric material to be heated are located along the resonant line, is dependent upon and takes into account the inter-electrode capacitances and the distributed lead inductances of the electron tube, which elements in and of themselves, may be considered to form a delay line of a predetermined length with the length thereof being determined by the values of the lead inductances and inter-electrode capacitances. In view of the fact that the impedance of a quarter-wavelength line with the far end open-circuited as viewed from its opposite end is zero, if such a line is connected across the anode and grid terminals of the electron tube the same effect that would occur if said terminals were short-circuited takes place. If the line length is more contracted than the above mentioned quarter-wavelength, this equivalently short-circuited point may be made to fall physically within the physical confines of the electron tube, with the result that a frequency higher than the upper limit frequency of the above mentioned alternative approaches can be generated. Thus, by locating the electrodes which receive the dielectric material to be heated at a predetermined position along the extra-tube transmission line the quarter-wavelength distance frorn the location of the electrodes in a direction of theV electron tube can be made to fall inside of the tube and hence cancel out a portion of the plate and grid lead inductances of the electron tube. The length of the extra-tube transmission line may be fully contracted until its length is restricted by the physical dimensions of the electron tube electrodes. With this arrangement, the pulling phenomenon can be completely avoided since the dielectric heating oper-ation can now be accomplished with the oscillating circuit only without providing a secondary circuit as was discussed with respect to one of the conventional alternatives presently in use.

It is therefore one object of the instant invention to provide heating means for dielectric materials employing novel ultra-short-wave techniques capable of highly stable heating at relatively high efficiency.

Still another object of the instant invention is to provide heating apparatus for dielectric materials which employs ultra-short-wave techniques wherein said heating apparatus employs a resonant line which is so designed as to be relative-ly unaffected with regards to its operating frequency upon insertion of the dielectric material into the receiving electrodes.

Another object of the instant invention is to provide novel heating apparatus for dielectric materials comprised of self-oscillating electron tube means coupled to a resonant line and having a pair of electrodes for receiving the dielectric loadA to be heated with the electrodes being positioned to cancel out a portion of the electron tube lead inductances in order to achieve higher frequency operation over prior-art devices and hence to obtain a more eflicient heating apparatus.

These and other objects of the instant invention will become apparent when reading the accompanying description and drawings in which:

FIG-URE l is a schematic diagram showing one priorart heating apparatus.

FIGURE 2 is a schematic diagram showing an alternative design for a prior art heating apparatus.

FIGURE 3 is a schematic diagram of the heating circuit of the instant invention showing the distribution of lead inductances and inter-electrode capacitances.

FIGURE 4 is an equivalent circuit of FIGURE 3, further showing the voltage and current distribution curves.

FIGrURE 5 shows a portion of the circuit of FIGURE 3 for the purpose of explaining the effect obtainable in the heating appara-tus of the instant invention.

FIGURE 6 is a schematic diagram showing the heating apparatus of FGURE 3 in greater detail.

FIGURE 7 is a figure showing partially in perspective the apparatus ofthe instant invention which was found to yield all of the objectives of the instant invention.

FIGURE 1 shows one prior art dielectric heating apparatus designated generally by numeral 10, which is comprised of a resonant line 16 having its rst end 16a shortcircuited and its second end 16b connected in parallel with the electron transmitting tube l5. This one-turn resonant line is inductively coupled to a 'second resonant line 17, which is short-circuited at end 17a and which is provided with a pair of electrodes lit for receiving a dielectric load 19 therebetween. The dielectric material 19, which is placed between the two plates 18 effectively form a tank condenser. The disadvantage of the arrangement 10 is that the value of Q Vof the dielectric load is quite high and the loaded Q of the secondary circuit becomes much higher than the loaded Q of the primary circuit due to the transformer effect between the two resonant lines resulting in the pulling phenomenon which is characterized by frequency skipping and unstable operating conditions as changes occur in the load during the course of the heating.

FIGURE 2 is a schematic diagram showing an alternative prior-art device comprised of electron tube 15 having delayV line 16 coupled thereto. The right-hand end 16a of delay line 16 is short-circuited, while its left-hand end 16b is coupled in parallel with the grid and anode electrodes of electron tube 15. The dashed and solid lines 13 and 14 plotted immediately above resonant line 16 of the condenser 11 into the apparatus which can be seen from the waveforms 13 and 14 showing the voltage and current distribution along resonant line 16. It can be seen that line 16 is a quarter-wavelength line so that decreasing and 14, respectively. Measured from the locations of the plate and control grid electrodes, the lead inductances convert into a resonant line having an equivalent length 1 corresponding to the sumkof Lp-l-Lg, where The equivalent length 1 is also shown in FIGURE 4. The total resonant line length then becomes the sum of the length of eXtra-tube transmission line 16, plus the equivalent transmission line due to the lead` inductances the lengththereof means a decrease below the quarter- Lp and Lg. Hence, the total effect of resonant line length wavelength dimension. The decrease in the length of line S gVn by L=1l1 Where 1/4 1l1' 1/2 n 16, as `previously mentioned, is further necessitated due Thus, by Very ludiciously IOCHHg the elecffdesls, to the presence of lead inductances of the electron tube, the point 21 (see FIGURE 4) which is a distance of oneinterelectrode capacitances within the electron tube and 15 quarter Wavelength from the OpeflrCfCUied 6nd and the inductances 0f the leads 17 connecting the eleetrgdes` which represents the short-circuited point, falls inside of electron tube 15 to the resonant line 16. In the eX- the tube @nl/610D@ 12 '[0 Cancel Out a Portion 0f the lead treme cases, the length 0f the resonant line which esinductances Lp and Lg. This is shown in equivalent cirtablishes a sh-ort-circuit between the anode 11b and grid Cuit faShOIl 11 FIGURE 5 by the ShOf'fng Strip 22 Which 11a electrodes of the tubes at the envelope wall 12 there- 20 S ShOWD 0 Short-Circuit a POTOU 0f the lad ndUCaHCeS of often determines the operating frequency for the oscil- Lp and Lg- Thus, the Same CI'TSC S Obtained 3S if these lator. The contraction of the length of resonant line 16 points were actually physically short-circuited. In this must be performed in order to raise the generated fremanner the extra-tube transmission line length may be quency.V When the physical dimensions of the electrodes fully COIlfaCd Until S lengh may n0 longer be further 18 are taken into consideration, the high voltage distribu- 25 contracted due to the restriction imposed upon further tion point 13a moves closer to thetube envelope 12 until contraction due to the physical dimensions of the eleca point is reached at which it is no longer possible to trodes 18. The high voltage distribution point is utilized connect the electrodes to the electron tube electrodes 11a by the installation of the electrodes 18 and the insertion of and 11b, thus preventing any further increasekin the op the dielectric load therebetween. The arrangement of erating frequency, Itis found that these practical limita- FIGURE 3 provides operation at a frequency substantions prevent the embodiment of FIGURE 2 from providtially higher than those obtainable through the apparaing sui'licient heating of the dielectric load. tuses shown in FIGURES l and 2 and it has been found In accordance with the instant invention, the resonant that the pulling phenomenon is completely dispensed line 1 c an be made appreciably long and accompanied with` with this, power can be generated up to the maximum To further prove the effectiveness of the circuit of the frequency determined by the electron tube without subinstant invention exhaustive tests were performed. A loop jecting restrictions upon the system which are imposed circuit of the type shown in FIGURE l was constructed. by the external circuitry. In addition thereto, the oscil- The relative positions and connecting methods for the cirlatorV can be loaded without a concern for the secondary cuit elements of the circuits were changed in many ways circuit shown in FIGURE 1. Extremely stabilized heat- 40 and a number of loops different in form such as the ciring characteristics are thereby obtainable by virtue of the Clllal IOOP, arc, and Straight lille Were adOped-` The instant invention. transmitting tube having the specifications shown in the FIGURE 3 shows the heating apparatus of the instant Table C was used. The generated frequency was measinvention which is comprised of an electron tube 15 of ured by an absorption type frequency meter. The highest the lself-oscillating type having plate, control grid and frequency obtainable for the 0.3 turn (ic, 108 angle) cathodeelectrodes P, G, and K, respectively. The resoloop was 70 megacycles per second. The following Table nant line 16 is coupled to the plate` and grid terminals A shows the comparison between this example and that p and g, respectively, has a line length 1, and is provided with a one-turn loop:

TABLE A Frequency, D.C. anode D.C. anode Input Pin Grid resist- D.C. grid D.C. grid Load out- Efficiency i No. of turns t. (mc/s.) voltage, current, (kw.) ance, R,i current, voltage, put, Pu 11 (percent) El, (kv.) Ib (a) (KS2) Ic (ma.) Ec (v.) (kw.)

0.3 70 2.9 0.6 1.74 6 so y 500 1.1 as 1.0 o4 4.1 0.39 1.60 e 60 380 1.0 63

at its far end with a pair of electrodes 18 for receiving dielectric load 19. vThe inter-electrode capacitances which are present are the plate to grid capacitance Cpg; the grid to cathode capacitance Cg'k; and thef'plate to cathode capacitance Cpk. The two lead inductances present are the plate lead inductance Lp and the Vcontrol grid lead inductance Lg. FIGURE 5 shows the equivalent circuit for these inductances and capacitances wherein the equivaient capacitance is equal to CpkCck Referring to FIGURE 4, together with FIGURES 3 and 5, the plate and grid locations are identified in FIGURE 4 by the designations P andG, respectively. The composite of the lead inductances Lp-l-Lg has been converted in FIGURE 4 to a length of resonant line having the same characteristic impedance as the extra-tube transmission line 16 and the voltage current distribution curves 13 The value of the lead inductance Lft-Lg obtained from the experimental data was 16.7 microhenries. Since the self inductance` of the single loop was 0.6 microhenry, it was proven that the generated frequency was substantially determined by the lead inductances and the inter-electrode capacitances. The high frequency operation desired was found to be unattainable using a circuit of the type shown in FIGURE 1.

FIGURES 6 and 7 show the set-up of the circuit (designed in accordance with the principles of the instant invention) employed to prove the effectiveness of the invention. The equivalent line length L was equal to V2A which was equal to 1.58 meters. The actual measured length of the resonant line was equal to 0.65 meter. The Teilon plate capacitor had a thickness of 0.5 millimeter. The eXtra-tube line length was varied in several ways to measure the generated frequency. A dielectric load comprised of a phenol plate was loaded between the electrodes at the open end of the line.

With the physical structure substantially as shown in the pictorial diagram of FIGURE 7, is was possible to generate an oscillating frequency of 95 megacycles per second. The output efficiency was found to be 73% from the measurement of load temperature rise. The specifics of the experimentation are shown in the following Tables B and C:

tube exhibiting inter-electrode capacitances and distributed leadl inductances; a pair of electrodes connected to said resonant line at said first end of said resonant line; the point of connection being the maximum H.-F. voltage distribution of said resonant line which is at or close to the open-circuited end thereof, and means for inserting a dielectric to be heated between said pair of electrodes in TABLE B Actual line Generated D.G. anode D.C. anode Grid D.C. grid D C. grid Input, Output, Efficiency, 1, length, l (in.) frequency, voltage, current, resista-nce current, i oltage, Pn. (kw.) P., (kw.) (percent) f. (mo/s.) Eb (kv.) Il, (a.) R1 (KQ) Ie (ma.) Ecfv.) l 0.65 95 5.6 I 0.24 15 39.5 -550 1.35 1.0 73

TABLE (Dr-SPECIFICATIONS OF THE TRANSMITTING TUBE Filament Electrostatic Capacitance Max. Ratings Mutual conductance, Amplication Voltage, Current, Gm (mv.) fantonp Cm, Cpt.- Clk D.C.anode D.C. grid Kind Ef (v.) If (a.) (mmf.) Y (mmf.) (mmf.) voltage, voltage,

En (kv.) Ee (v.)

Thorium tungsten 7.5 21 4.5 (lb=0.3a) 14,3 6.8 0.3 9.9 7.5 1,000

Filament Max. Ratings External Dimensions DC. D.C, Anode Anode Grid Total Max. Kind Voltage, Current, anode grid input, dissipation, dissipation, length diameter Ef (v.) I; (a.) current, current, Pr (kw.) 1) (kw.) Pg (w.) (min.) (mm.)

Is (a.) It (a.)

Tliorium tungsten 7.5 21 0.75 0.125 4 1 80 250 142 It can clearly be seen that the instant invention provides a self oscillation heating apparatus capable 4of producing a generated frequency which is appreciably higher than conventional devices and which is totally unaffected by the pulling phenomenon even though the load may undergo substantial changes during the heating operation. The markedly increased frequency of operation further led to an increase of in the efficiency of the ap- 4 paratus.

Although an embodiment of the invention has been described herein in connection with a single tube self oscillation circuit, it will be understood that there are a number of modifications all lying within the scope of the instant invention. For example, the invention applies equally to circuits in which two or more electron tubes are connected in parallel operation or in push-pull operation. Also, while the preferred embodiment described herein teaches an apparatus in which the effective resonant line length L is given as V2A, it should be understood that any integral multiple (N+2)l\ of resonant line length may be employed.

Although preferred embodiments of the novel invention have beeny described, many variations and modifications will now be apparent to those skilled in the art, and it is preferred therefore to be limited not by the specific disclosure herein but only by the appended claims.

The embodiments of the invention in which an exclusive privilege or property is claimed are defined as follows:

1. Ultra-sbort-wave dielectric heating apparatus comprising a circuit provided with a resonant line having a first end open-circuited or nearly open-circuite'd; an electron tube having cathode, anode and grid electrodes; said anode and grid electrodes being connected to said line at the sec-ond end thereof to Y form a self oscillation circuit with said resonant line; said electron such a manner that said open-circuited state may not be affected; said point of connection being located at a disstance of less than 1/4 wavelength of the operating frequency from the electron tube envelope thereby moving the point of maximum current and zero voltage of said transmission line to the tube interior to cancel a portion of said lead inductances.

2. The device of claim l wherein the length of the external resonant 1ine=1; the effective intra-tube resonant line l'=Lp-l-Lg where Lp=the plate lead inductance and Lgzthe control grid lead inductance; where References Cited by the Examiner UNITED STATES PATENTS 2,563,098 8/1951 Brown 2l9-10.53

FOREIGN PATENTS 667,657 3/1952 Great Britain.

OTHER REFERENCES Radar Electronics Fundamentals, Bureau of Ships, Navy Dept., Washington, D.C., June 1944.

RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, Assistant Examiner, 

1. ULTRA-SHORT-WAVE DIELECTRIC HEATING APPARATUS COMPRISING A CIRCUIT PROVIDED WITH A RESONANT LINE HAVING A FIRST END OPEN-CIRCUITED OR NEARLY OPEN-CIRCUITED; AN ELECTRON TUBE HAVING CATHODE, ANODE AND GRID ELECTRODES; SAID ANODE AND GRID ELECTRODES BEING CONNECTED TO SAID LINE AT THE SECOND END THEREOF TO FORM A SELF OSCILLATION CIRCUIT WITH SAID RESONANT LINE; SAID ELECTRON TUBE EXHIBITING INTER-ELECTRODE CAPACITANCES AND DISTRIBUTED LEAD INDUCTANCES; A PAIR OF ELECTRODES CONNECTED TO SAID RESONANT LINE AT SAID FIRST END OF SAID RESONANT LINE; THE POINT OF CONNECTION BEING THE MAXIMUM H.-F. VOLTAGE DISTRIBUTION OF SAID RESONANT LINE WHICH IS AT OR CLOSE TO THE OPEN-CIRCUITED END THEREOF, AND MEANS FOR INSERTING A DIELECTRIC TO BE HEATED BETWEEN SAID PAIR OF ELECTRODES IN SUCH A MANNER THAT SAID OPEN-CIRCUITED STATE MAY NOT BE AFFECTED; SAID POINT OF CONNECTION BEING LOCATED AT A DISSTANCE OF LESS THAN 1/4 WAVELENGTH OF THE OPERATING FREQUENCY FROM THE ELECTRON TUBE ENVELOPE THEREBY MOVING THE POINT OF MAXIMUM CURRENT AND ZERO VOLTAGE OF SAID TRANSMISSION LINE TO THE TUBE INTERIOR TO CANCEL A PORTION OF SAID LEAD INDUCTANCES. 